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A study of laboratory mice published in Thursday's journal Nature suggests that the absence of a specific gene makes them more vicious and sexually agressive than normal mice. "We can't concretely make any conclusions about violent behavior in humans from this study," says TIME's Alice Park. "But what's hopeful and promising is that this particular gene is very similar in mice and in humans. But something as complex as violent behavior in human beings obviously can't be traced to a single gene." The scientists from the Johns Hopkins University School of Arts and Sciences and Massachusetts General Hospital bred the mice to eliminate a working copy of a gene needed to make an enzyme called nitric oxide. The researchers say that nitric oxide may act as a brake on behavior.